The invention relates to a method for identifying geometric deviations of a real motion guide from an ideal motion guide in a coordinate-measuring machine having a sensor for measuring a workpiece, or in a machine tool having a tool for processing a workpiece, wherein the coordinate-measuring machine or the machine tool has a part which is movable along the motion guide. The part can be in particular one where, during its movement, the sensor or the processing tool is moved. Alternatively, it can be a part which can be moved independently of the sensor or the processing tool. One example is a rotary table on which the workpiece that is to be measured or processed is arranged. The invention furthermore relates to an arrangement for identifying such geometric deviations in a coordinate-measuring machine or in a machine tool. The invention additionally relates to a coordinate-measuring machine or to a machine tool having said arrangement. The motion guide can be for example a linear guide for guiding the movement of the part along a linear axis. Alternatively, the guide may for example be one which guides the movement of the part about a rotary axis.
Coordinate-measuring machines (CMM) typically have at least one sensor which is movable relative to a workpiece that is to be measured. The sensor can be a probe, for example a stylus, with which the workpiece is probed while being in contact therewith, i.e. in tactile fashion. The sensor can be arranged at a measuring head which provides in accordance with the tactile probing measurement signals, from which, in particular in combination with current positions of the movable parts of the CMM, the coordinates of the probed workpiece are calculated. Alternatively, the sensor can be a probe of the switching type. The sensor can, as a further alternative, be a different sensor, for example an optical sensor or a capacitive sensor. In machine tools, at least one processing tool is present with which a workpiece can be processed.
Frequently, the sensor or the processing tool is movable with the result that the position and/or alignment thereof relative to the workpiece is settable. Alternatively or additionally, a holder or support for the workpiece is movable, with the result that the position and/or alignment thereof relative to the sensor or the processing tool is settable. Examples are measurement tables which are movable in two mutually perpendicular linear directions (so-called cross tables) and rotary tables. In all these cases, motion guides are used with the purpose of performing the movement reproducibly along the movement path defined by the respective motion guide.
Guidance defects of the motion guide can be in particular production defects of the motion guide and/or be caused by deformations of the motion guide during operation. Guidance defects can be given in accordance with the six degrees of freedom of the movement of a body as linear guidance defects with respect to the three axes of a Cartesian coordinate system and as rotational guidance defects with respect to said three axes. For measuring these components of the guidance defect, one or more measurement methods and corresponding measurement devices can be used.
DE 10 2012 207 388 A1 discloses a method for ascertaining geometric defects of a coordinate-measuring machine. A sensor arrangement held by the CMM records measurement values of the geometric defect by moving and measuring a planar calibration surface having raised edges, which is arranged on the movable measurement table of the CMM. The present invention can also use measurement values which are recorded in this manner.
For correcting the guidance defects, it is known in coordinate-measuring machines (CMM) to use the CAA (computer aided accuracy) method. In a calibration, measurement values of the geometric deviation of the real motion guide (i.e. in the actual state of the motion guide during the calibration) from an ideal motion guide (i.e. the motion guide as it would be without guidance defect, for example exactly linear or exactly in the form of a circular line) are recorded. Corresponding correction values are ascertained therefrom and provided in a data store for correction purposes. The guidance defect is recorded, for example, using a measurement device having a laser interferometer or, specifically in the case of torsion guidance defects, using a measurement device having electronic inclination sensors. Ball plates and/or perforated plates or other calibration standards, which can be scanned in particular by the sensor of the coordinate-measuring machine, can also be used for recording the guidance defects. For correcting elastic bending portions of the guides, the so-called surface CAA is also known.
The CAA correction model is based on the assumption that the guidance defects are stable in the long run, i.e. are the same at the time when the defect is recorded and at the time when the workpiece is measured, with it being possible for a several months or years to have passed between said time points.
In coordinate-measuring machines and machine tools, guidance changes also occur, however, over time, for example due to foundation subsidence effects, and during operation, for example due to weight forces of the workpiece or due to temperature changes. These effects are dependent on time and are not stable in the long run like the guidance defects corrected by way of the CAA. They are referred to as dynamic guidance changes or dynamic guidance defects. For this reason, there is a need for repeating or at least checking the calibration. The outlay herefor should be low, since otherwise less time for operating the machine or the apparatus remains.